Significant doses of nerve agents are deadly, but it is unclear whether there are long-term effects of exposure to smaller amounts.

Dr Mirzyanov, who has not worked in Russia for 30 years and has not been approached by the British authorities for advice, said he believes trace elements could lead to complications similar to Gulf War syndrome, which has been attributed to exposure to pesticides and chemicals released during that conflict.

Public Health England (PHE), the agency responsible for responding to health emergencies, has consistently advised that the risk to the general public is low, and repeated that position in response to his claims.

Dr Jenny Harries, PHE’s deputy medical director, said: “PHE has been working very closely with the police and national experts on chemical weapons since the start of the incident and our risk assessment is based on knowledge of the chemical used. Our advice remains that the risk to the general public is low.”

Most significantly, on Sunday it advised anyone who was in the pub or restaurant visited by the Skripals to wash clothing and items such as watches, phones and spectacles.

PHE said the measures were precautionary and the immediate risk was “extremely low”, but it drew criticism from figures including former chief medical officer Liam Donaldson, who questioned whether its response had been too slow.

Officials are adamant the reaction has been proportionate, timely and underpinned by the very best intelligence and expertise.

PHE retains experts in biological and chemical weapons, and manages a rare and imported pathogens laboratory on the same site at Porton Down as the Defence Science and Technology Laboratory, where the novichok samples were analysed.

PHE officials have worked closely with the police and their experts at Porton Down, and it is understood they were informed novichok had been used as soon it was identified, perhaps within 24 hours of the attack.

Their subsequent assessment of the wider health threat was based on three factors: the dose used in the main attack, the trace amounts identified, and “proportionality” of response.

Initially, with no traces of the compound found, they advised the public that there was a low risk while the emergency and health response focused on assessing medical staff and first responders – including Detective Sergeant Nick Bailey, who remains in hospital in a serious condition.

Last Sunday, however, the advice was adapted when it emerged that trace elements had been found at the Zizzi restaurant and The Mill pub.

Officials insist their position has developed appropriately as the facts have changed, and believe there is a very small risk from repeated exposure to small amounts of novichok.

They maintain that dilution with water is the most effective way of dealing with trace elements, hence the advice to wash clothes and personal items, and that the risk is limited to the small number of people who visited the two sites.

PHE continues to review its advice but privately officials believe they have been vindicated by the fact that no one else in Salisbury has been affected.

“If people had been affected we would already know about them,” said one source. “Bluntly, they would be in hospital.”